A construction machine such as a hydraulic excavator uses gasoline, light oil, or other fuel as a motive power source and drives a hydraulic pump with an engine to generate hydraulic pressure, thereby driving hydraulic actuators such as hydraulic motors or hydraulic cylinders. As the hydraulic actuator is compact, lightweight, and capable of generating high output, it is widely used as a construction machine actuator.
Meanwhile, a construction machine recently proposed, for instance, in Patent Document 1 uses an electric motor and electrical storage devices (e.g., a battery and an electric double layer capacitor) to provide increased energy savings by achieving higher energy efficiency than a conventional construction machine that uses only hydraulic actuators.
The electric motor (electric actuator) excels in terms of energy conservation as it achieves higher energy efficiency than the hydraulic actuator and can generate electrical energy from kinetic energy during braking (the kinetic energy used during braking is released in the form of heat when the hydraulic actuator is used).
For example, Patent Document 1, which relates to a conventional technology, describes an embodiment of a hydraulic excavator that incorporates an electric motor as a swing structure drive actuator. An actuator that turnably drives an upper swing structure of the hydraulic excavator with respect to a lower track structure (a hydraulic motor is conventionally used) is frequently used as it starts, stops, accelerates, and decelerates repeatedly at frequent intervals.
In this instance, the kinetic energy of the swing structure that is used during deceleration (braking) is discarded in the form of heat within a hydraulic circuit when the hydraulic actuator is used. However, when the electric motor is used, energy saving can be achieved because the kinetic energy can be regenerated as electrical energy.
In the meantime, a construction machine proposed, for instance, in Patent Documents 2 and 3 incorporates both a hydraulic motor and an electric motor and drives a swing structure by using the total torque generated by the two motors.
An energy regeneration device disclosed in Patent Document 2 is for use in a hydraulic construction machine in which an electric motor is directly coupled to a swing structure drive hydraulic motor and a controller issues an output torque command to the electric motor in accordance with the amount of control lever operation. During deceleration (braking), the electric motor generates electrical energy from the kinetic energy of the swing structure and stores the generated electrical energy in a battery.
A hybrid construction machine disclosed in Patent Document 3 calculates a torque command value for an electric motor in accordance with a differential pressure between the inlet and outlet of a swing drive hydraulic motor and allots an appropriate output torque to each of the hydraulic motor and the electric motor.
Conventional technologies described in Patent Documents 2 and 3 both use an electric motor and a hydraulic motor simultaneously as swing drive actuators so that an operator familiar with a conventional, hydraulic-actuator-driven construction machine feels comfortable when performing operations, and that increased energy savings will be realized by using a simple, easy-to-implement configuration.